SEP Transition to Simple with Funding

Hello. I have a client with an existing SEP Plan (prototype w/ TD).  I know we can still make a 2020 contribution of course, but not sure on this:  Can a 2020 contribution be made to the SEP, we then terminate the SEP, and finally establish a Simple for 2021?  If so, does the SEP termination need to be retro to 12/31/2020, and is that even possible?

Thank you in advance.

You can fund the SEP for 2020 and establish the SIMPLE for 2021.  No need to terminate the SEP; I'm not sure that has meaning.  Just don't fund it going forward.

Thank you.